To Charles Darwin it was emblematic of sexual desirability. To French naturalists it was "grossly monstrous". But even to the uneducated eye, it was conspicuous as an alarmingly oversized appendage.

The toucan's bill is one of the most bizarre products of evolution, but in the past 200 years scientists have failed to agree why it has grown so huge. Some argue it helps the birds collect hard-to-reach fruit, while others say it is a warning to rivals, or helps them raid other birds' nests for food.

Research by Canadian and Brazilian scientists puts forward a completely new theory that helps explain how the toucan got its bill. In a report in the journal Science, they claim the appendage doubles as a giant radiator that keeps the birds cool in the heat of the tropics.

The thermal camera revealed that the birds use their bills to control their body temperature by adjusting blood flow into the appendage. By opening or closing blood vessels in the beak, the birds can lose as little as 5% or as much as 100% of their body heat through their bill.

The study puts toucans' beaks on a footing with elephants' and rabbits' ears as nature's solution to life in a hot climate.

Thermal images of the birds show that at sunset, as they were preparing for sleep, their bills cooled by around 10C in a matter of minutes. The large, exposed beak makes up around 40% of their overall surface area, so it rapidly radiates body heat and helps them to fall asleep. Immediately before nodding off, the birds cover their bills with their wings.

Tattersall describes in Science how the bill might also help the birds cool down after the exertion of flying. One bird in the study warmed up from 31C to 37C within 10 minutes of taking to the air. "When the blood vessels in the bill are dilated, the bird can lose nearly five times as much heat as they produce," Tattersall said.

He suspects that other birds use their bills as cooling systems too, which might explain why birds in polar regions tend to have smaller beaks than those in warmer climates.